DDR3 RAM has become a lot more better in value than it was few months back. Now this is the right time to get a pair or two of DDR3 memory for your system specially since DDR2 price has gone up and close to DDR3 prices.
DDR3(Double Data Rate 3) is a newer, faster and most advanced technology in system RAM but…… yes there is a but, DDR3 is not always faster than DDR2, specially when it’s lower in speed, lets say you have DDR2 running at 1066MHz and DDR3 running at same 1066MHz speed results will be very disappointing from DDR3 prospective.
Because at 1066MHz speed DDR3 RAM will run at cas 9 timings whereas DDR2 at same speed will be running at cas 5 timings and you just can’t run DDR3 at 1066MHz with cas 5 timings and if somehow you succeed in running DDR3 1066MHz at cas-5 timing you will have stability issues.

Bandwidth And Latency Times!
To give you better understanding of what is bandwidth and what is latency timing here is a short note:Bandwidth:
1. This is the SPEED at which memory works, higher bandwidth means faster data transfer.
2. A 1066MHz speed memory goes faster through clock cycles than 800MHz speed.Cas Latency (tCL):
What is Cas Latency? it’s very simple to understand it:
1. This is the TIME (in clock cycles) it takes between arrival of request for the data and it being delivered, for example you wait if you RAM is CAS 2 it will take 2 clock cycles to perfom the task.
2. The request is sent from the memory controller to the memory module.

I am sure that now you have the understanding of how memory works, higher bandwidth memory works faster but higher latency times takes longer to fetch data, so it will make a 1066MHz DDR3 look no faster and in some cases even slower than a DDR2 800MHz, that’s why you see memories with lower latency times priced higher than same speeded memories with higher latency times.

Power Efficiency:
DDR3 is power efficient than DDR2, DDR3 at higher bandwidth performs faster than DDR2 and consumes less power than DDR2. DDR3 requires 1.5V to properly run whereas DDR2 has the voltage requirement of 1.8V.

DDR2/DDR3 Hybrid Boards!
If you are one of those lucky ones who have a DDR2/DDR3 hybrid board, a board that has both DDR2 and DDR3 slots and you want to upgrade your RAM you better stick with DDR3 instead of DDR2, the only reason I can give you is the price at the moment, these days price is playing a big role in memory. If you already have DDR2 RAM then sell them and upgrade by getting a pair of DDR3 RAM.Consider these DDR3 RAMs seen on this page!

Faster Speed DDR3 Memory!
By faster speed I mean higher bandwidth memory.
When buying system memory it makes sense when you pay just 20 to 30 dollars more to get a 4GB kit of higher speed DDR3 RAM than a 4GB kit of DDR2 running at 1066MHz. Always go for a faster pair of DDR3 memory because a DDR3 at speed of 1066MHz with timings set at 9-9-9-18 will perform worse than a set of DDR2 running at same 1066MHz speed with 5-5-5-15 timings.
Slower latencies is the only reason to avoid lower speed DDR3, price used to play an important role but not anymore because today almost all RAM companies has increased DDR3 production which result in decreased prices for DDR3 RAM and increased prices for DDR2 RAM, but the biggest draw back is slower latencies.
So I suggest you to stay away from entry level DDR3, entry level DDR3 doesn’t perform on par with high level DDR2 and you can’t compare entry level DDR3 with enthusiast level DDR2 because the performance difference will be too much just because of latency timings.

Conclusion!
The downside of DDR2 is bandwidth and upside is latency timings, whereas the downside of DDR3 memory is timings and upside is bandwidth, so if you go for DDR3 make sure it’s higher bandwidth DDR3 to be on safe side.
So before you invest into very high priced DDR2 think twice. If you are building a totally new rig I will highly recommend you to go with DDR3 system. There isn’t much difference in a kit of 4GB DDR2 and DDR3 RAMs. You can even find 4GB kit of some enthusiast level DDR3 RAMs running at 1800MHz speed for the same price of DDR2 4GB kit running at 1200MHz speed or lower 1066MHz speed. Make sure you keep these points in mind before you upgrade your system memory.
So don’t be a fool and invest wisely.